The present disclosure relates to the processing of silicon wafers in phosphoric acid solutions. In particular, it provides a novel method to prevent the growth of colloidal silica deposits on the wafer surface during processing in phosphoric acid.
Phosphoric acid has been used in the semiconductor industry to remove silicon nitride films with high selectivity to silicon dioxide and pure silicon. In 1967, Gelder and Hauser published an article where they proposed the use of boiling phosphoric acid to remove silicon nitride films. They used the boiling point of the acid at a given dilution to control the acid temperature and its concentration in the process tank. This process has been widely adopted in the semiconductor industry and it is used in production today.
A typical application of the boiling phosphoric acid process in the semiconductor industry is the removal of silicon nitride films. In such applications, the silicon nitride film typically sits on a silicon dioxide film and it is surrounded by deposited silicon dioxide. The process for the removal of the silicon nitride film begins by a careful removal of any remaining silicon dioxide that might rests on top of the silicon nitride film by a diluted hydrofluoric acid (HF) treatment. The dilution of the HF is tailored to remove any remaining silicon dioxide from the surface of the silicon nitride without significantly removing the deposited silicon dioxide that was placed between the silicon nitride regions. After the silicon nitride deglaze step in HF, a high selectivity etch is performed in boiling phosphoric acid to remove the silicon nitride film without removing the deposited silicon dioxide film that exists between the silicon nitride films, stopping on the silicon dioxide film that lays under the silicon nitride film.
It has been found that during processing of certain semiconductor structures the adverse growth of colloidal silica deposits on the exposed silicon dioxide regions may hinder silicon nitride removal and/or other subsequent process steps. There is a need for a method to remove the colloidal silica deposition during the treatment of surfaces in phosphoric acid.